Over the past decade, much research has been devoted to developing new and useful devices for delivering beneficial agents to agent receptor environments of use. For example, in U.S. Pat. No. 3,760,984 issued to Theeuwes, there is disclosed an osmotic delivery device comprising an inner collapsible container carrying on its outer surface a layer of an osmotic solute and a surrounding layer of a polymer permeable to fluid and impermeable to solute. In U.S. Pat. No. 3,971,376, issued to Wichterle, a device is disclosed comprising a capsule having a unitary wall formed of a substantially noncollapsible elastic material that maintains a constant volume and which is adapted to be implanted subcutaneously. A textile fabric may be imbedded in the capsule wall. The fabric strengthens the wall and acts as a reinforcement. In U.S. Pat. No. 3,987,790 issued to Eckenhoff et al., there is disclosed another osmotic delivery device which contains an outer shape-retaining membrane which is sufficiently rigid to be substantially undeformed by the hydrostatic pressure exerted by water permeating through the membrane.
U.S. Pat. No. 3,995,631 issued to Higuchi et al., discloses a device (FIG. 4) comprising an inner flexible bag containing a drug formulation. The bag separates the drug from an osmotically effective solute material. Both the drug and the solute are contained within a housing having an exterior wall that is, at least in part, semipermeable. U.S. Pat. No. 3,995,632 issued to Nakano et al discloses a similar device which incorporates a movable barrier within the housing. The barrier divides the housing into two compartments, one containing the solute and the other containing the drug. The solute-containing compartment has an exterior wall that is, at least in part, semipermeable. This compartment acts as an osmotic driver for the device. U.S. Pat. No. 4,410,328 issued to Theeuwes discloses an osmotically driven syringe/pump device. The osmotic driver in this device is in the form of a tablet comprising an osmotically effective solute, such as sodium chloride, within a semipermeable wall having a single exit orifice drilled therethrough.
While the above-described devices are useful for delivering many agents, and while they represent a valuable contribution to the delivery art, there has been a need in the art for an osmotically driven syringe/pump utilizing an osmotic driver which can be easily replaced and which can be mounted in an osmotically driven syringe/pump in a fluid tight manner. Unfortunately, the osmotic drivers utilized in the prior art devices have had poor strength and shape-retaining characteristics. These osmotic drivers have typically been in the form of a tablet of an osmotically effective solute (e.g., sodium chloride, lithium chloride, potassium chloride, sodium sulfate, and the like) coated with a thin layer of either a semipermeable or microporous membrane material. Known osmotic drivers were made by compressing the solute into the shape of a tablet and then suspending and tumbling the tablet in a wall-forming composition until a thin membrane wall is formed around the solute. Next, after drying, a passageway is drilled through the wall. The air suspension procedure is described in U.S. Pat. No. 2,799,241; in J. Am. Pharm. Assoc., Vol. 48, pages 451 to 459, 1959; and J. Am. Pharm. Assoc., Vol. 49, pages 82 to 84, 1960. Other wall forming techniques such as pan coating have been used in which materials are deposited by successive spraying of the polymer solution on the solute, accompanied by tumbling in a rotating pan. Generally, the semipermeable wall will be about 0.5 to 50 mils thick.
The semipermeable membrane of the prior art osmotic engines have typically been made from materials such as cellulose acylate, cellulose diacylate, cellulose triacylate, cellulose acetate, cellulose diacetate, cellulose triacetate, and the like. Unfortunately, when these membranes are exposed to water, they tend to soften, weaken and expand due to hydration of the membrane. As the membrane-supporting solute core is dissolved and delivered by the osmotic driver, the driver begins to lose its shape. Once the solute has been completely dispensed, the semipermeable membrane collapses and takes the form of a soft amorphous mass.
U.S. Pat. No. 4,008,719 discloses an osmotic driver having a two-layer semipermeable wall formed of cellulose acetate polymer. Semipermeable walls of this type have a thin dense outer layer and a honeycombed supporting inner layer. The honeycombed layer provides some physical support for the thin outer layer. Unfortunately, the cellulose acetate membranes of the type disclosed in U.S. Pat. No. 4,008,719 possess neither great strength nor rigidity. The membranes typically have a Youngs modulus in the range of only about 1000 to about 5000 psi and a compressive strength at 10% compression of only up to about 100 psi. When the prior art osmotic drivers, having the above-described two-layer membrane wall structure, are hydrated, the membrane wall becomes soft and flexible. Once the driver becomes hydrated and has delivered part or all of its osmotic charge, a compressive pressure of only about 5 psi or less will deform the driver. Accordingly, the prior art osmotic drivers are unable to withstand the compressive stresses imposed by the design and operation of an osmotic syringe/pump according to the present invention.
Therefore, it is an object of the present invention to provide an osmotic driver, adapted for driving a fluid dispensing syringe/pump, and having good strength and good shape-retaining characteristics even after the driver has delivered part or all of its osmotic charge. In particular, it is an object of the present invention to provide an improved osmotic driver having a rigid internal reinforcing structure enabling the osmotic driver to retain its initial shape during use and enabling the driver to withstand the compressive stresses imposed by fixedly securing the driver within an osmotic syringe/pump, all without compromising the operation of the or the syringe/pump.